What are you eating under there?

Perhaps one of the greatest staples of grade schoolhumor, the "What are you eating under there?" joke has been passed amongst children for a long time, and presumably still is.

The joke goes like this: Bobby walks up to Billy on the playground and asks: "What are you eating under there?" Billy, of course, is not under anything, so asks quizzically: "Under where?". Which of course, sounds like "underwear". Then Bobby laughs at Billy and says "you are eating underwear?".